Apple says 10 hours. We won't know until the unit ships. We do know that battery is sealed (just like the MacBook Pros, iPhone, etc.) so removing it will cost you $99 with $6.95 for shipping. This will let you send Apple your iPad, and Apple will send you a different iPad. Yes, a different one so backup your data through iTunes first.

If you have a current AppleCare warranty, Apple will replace the battery free if the capacity drops below 50% during the first 2 years of battery ownership.